Fig. 5: Magnetism breaks local inversion symmetry enabling d–d transition.
From: Magnetically brightened dark electron-phonon bound states in a van der Waals antiferromagnet
With the onset of Néel order, the local environment around nickel sites breaks the local inversion symmetry and makes the electric-dipole forbidden d–d transition visible in the absorption spectrum. For T > TN, all sulfur atoms (yellow) are equivalent, and hence nickel site is a local inversion center. For T < TN, nickel atoms are no longer local inversion centers, as the magnetic order creates two sets of distinguishable sulfur atoms: the ones between the opposite spins (orange) and the ones between the parallel spins (yellow). These dissimilar sulfur atoms break local inversion at nickel sites, as shown on the right.
